The field of the invention is rotary drum vacuum filters used in the pulp and papermaking industry to form a mat of wood pulp and separate the mat from its filtrate. In particular, the invention relates to gas vent on the drum suction control valve in the discharge elbow assembly of the filter.
FIG. 1 shows a rotary drum vacuum filter 10 that includes a rotary drum 12 in a vat 14 of pulp slurry. The drum is partially submerged in a pulp slurry vat vessel, such as up to the horizontal centerline of the drum. The drum turns in a clock-wise direction at a preferred rate of approximately 2 to 4 revolutions per minute (RPM) and most preferably at 3 RPM. As the outer drum surface rotates through the slurry (3:00 to 9:00 positions), a pulp mat 16 forms on the outer face 17 of the drum.
To promote mat formation, suction is applied to the drum porous outer surface 17, e.g. a screened or wire surface. The porosity of the surface 17 is sufficiently fine to retain fibers on the surface and pass primarily filtrate into the channels 18 behind the porous surface. The channels 18 are arranged in a longitudinal array behind the screen and extending the length of the drum. The channels drain into radial channel 20, or tubes, that lead to a central filtrate chamber 28.
As the surface 17 of the drum travels up and out of the vat (corresponding to the 9:00 to 12:00 rotational positions of the drum), the pulp mat 16 on the surface is washed with a liquid spray 22, e.g., wash water, that cleans the pulp mat of chemical liquor. The suction draws the water and liquor from the pulp mat into the channels 18 behind the drum surface 17. The channels continue to drain into the channels 20 which drain into the filtrate chamber that is typically at one end of the drum and coaxial to the drum.
As the drum surface passes over the top rotational position (12:00 to 1:00), the wash water spray is stopped. As the drum rotates towards the 2:00 position, the suction stops, but water continues to drain through the pulp and into the channels and ribs. Air also starts to enter the channels and ribs because of the stoppage of wash water.
The concentrated pulp is generally referred to as a pulp cake. As the drum rotates through to the 2:00 to 3:00 position, a scraper 24 removes the pulp mat from the drum surface. The pulp cake is collected in a chamber 26 for further processing.
Vacuum washers typically receive a low consistency pulp slurry (1.5% pulp by weight) in the vat vessel. The pulp is thicken as the drum surface rises on the drum surface out of the vat to about a 10% consistency. The pulp is further thickened to a discharge consistency from the drum of 12% or greater.
After the cake is removed, the channels and ribs are typically filled with air. As the drum surface (now scraped clean of the pulp mat) rotates past the 3:00 position, the surface renters the vat 14. Suction is reapplied to the channels and ribs after the surface is submerged into the vat. A pulp mat 16 begins to form again on the drum surface 17. The formation of a pulp mat, water cleaning of the mat, and scraping of the map off the drum is a continuous process that occurs as the drum rotates.
The motive force for the suction on the drum surface is the vacuum created as the extracted filtrate drops approximately 30 feet (ft.) to 40 ft. (10 to 13 meters) from the rotary drum vacuum washer 10 to a filtrate tank (below the washer). The pipe through which the filtrate passes is known as a drop leg 32 (FIG. 2).
FIG. 2 shows a conventional end of a rotary vacuum filter having a drum 12 and a drainage path for liquor and wash water (collectively filtrate) that flows from the longitudinal channels 18 (FIG. 1) and radial channels to a filtrate chamber 28 typically at one end of the filter 10 and coaxial to the drum. Suction to the drum surface 17 is generally provided through the channels 18 that extend behind the screen on the drum face 17. Liquor and water (collectively “filtrate”) enter the channels and are drawn by suction into rib conduits that extend radially and partially axially from the channels near the drum face to an filtrate chamber 28 typically at one end of the drum.
The axial filtrate chamber 28 provides a drainage path for the flow of filtrate from the ribs and channel in the drum. The filtrate chamber 28 is traditionally coupled, (through a trunnion conduit 34 and an elbow joint 30), to a drop leg conduit 32 that drains the filtrate flow down below the vat 14 to a filtrate collection vessel (not shown).
The drainage of the filtrate into the drop leg 32 creates a suction that draws the filtrate through the filtrate chamber 28, ribs 20 and channels 18. To maintain high levels of suction, gas, e.g., air, should not flow into (or at least not become excessive) in the chamber 28, elbow 30 or drop leg 32. If too much air enters the drop leg, the suction level (sub-atmospheric pressure) lessens, the flow of liquid filtrate into the drop leg may be interrupted such that reduced suction will be applied to the filtrate chamber 28, ribs and channels and air enters the filtrate flowing through the drop leg which may cause the filtrate to foam and require downstream processing to remove the air. Accordingly, there is along felt need to prevent gas from entering the elbow joint 30 and drop leg 32.
FIG. 2 shows an exemplary prior art approach to preventing gas from entering the elbow joint 30 and drop leg 32. The filtrate chamber 28 in the drum 12 is coupled to a trunnion conduit 34 that rotates with the drum. The trunnion conduit 34 is driven through a worm gear 36 and a matching drive worm gear collar 37 to rotate the drum. The elbow 30 and down leg 32 conduits are stationary. An inlet end of the elbow is coupled to the outlet of the rotating trunnion conduit. FIG. 2 is an exploded view of the trunnion conduit and elbow and down leg. In practice, the outlet of the trunnion conduit is rotatably coupled to the inlet to the elbow conduit 30 and the elbow and down leg 32 conduits are connected.
A center shaft 38 extends from the elbow into the trunnion conduit 34. The center shaft is of a relatively small diameter as compared to the inner diameter of the filtrate passage in the elbow and down leg. The center shaft 38 is hollow to allow gases in the filtrate to vent into the shaft and avoid entering the filtrate passage in the elbow 30 and down leg 32.
The center shaft supports a valve segment 40 that includes a generally arc shaped section that extends from about the 1:00 position to the 5:00 position relative to the rotation of the drum. The outer face of the valve segment is positioned in the filtrate chamber and juxtaposed against the drainage outlets for the ribs 20 (as the ribs pass through the 1:00 position to the 5:00 position). The drainage outlets of the ribs open to the filtrate chamber 28.
The valve segment blocks the outlets of the ribs in the drum as the ribs rotate through the 1:00 to 5:00 positions. The arc width of a conventional valve segment is typically about 130 degrees which corresponds to rotating the drum through the 1:00 to 5:00 positions. The ribs are prevented by the valve segment from draining to the filtrate chamber 28 and into the trunnion conduit. As the ribs rotate from 1:00 to 5:00, filtrate and gases, e.g., air, in the ribs are intended to remain in the ribs. The valve segment 40 prevents most of the gases in the ribs from flowing into the filtrate chamber 28 and to the trunnion conduit 34, elbow conduit 30 and down leg conduit 32.
The valve segment 40 also prevents suction from being applied to the ribs as the ribs pass from the 1:00 to 5:00 positions. Suction is neither needed nor desired as the surface 17 of the drum passes from the 1:00 to 5:00 positions because gravity holds the pulp mat 16 on the surface until the scraper 24 (FIG. 1) removes the pulp cake 16 at about the 2:00 to 3:00 position. Suction if applied from the 1:00 to 5:00 positions would draw air into the channels and ribs and impede removal of the pulp mat.
The valve segment 40 does not block the application of suction to the ribs or the drainage of filtrate from the ribs as the ribs rotate from the 5:00 position to the 1:00 position. As the ribs move through the vat, suction (applied through the ribs by the down leg) draws a pulp slurry onto the drum face screen and pulls filtrate through the screen and into the channels, ribs and to the filtrate chamber 28. Similarly, as the ribs move up out of the vat to the top drum position (3:00 to 12:00), the suction draws filtrate, including the wash water, through the screen and into the channels, ribs and filtrate chamber. The flow of filtrate into the ribs moving from the 5:00 position to the 1:00 position is sufficient to create a substantial suction as the filtrate flows into the elbow conduit 30 and down leg conduit 32. Substantial amounts of air are prevented from entering the elbow and down leg because the channels and ribs are substantially filled with liquid filtrate as the channels are submerged in the vat and pass under the water spray, which occurs as the drum moves from the 5:00 position to the 1:00 position. After the channels rotate past the water spray (at about the 12:00 to 1:00 position), the outlets to the ribs are block by the valve stem to prevent gas from entering the filtrate chamber and trunnion conduit.
The valve segment 40 does not prevent all gases from entering the elbow and down leg. Air enters the ribs as the liquid filtrate drains from the ribs rotating from the 1:00 position until the channels for the ribs enter the vat. The air remains in the rib as the rib rotates down into the drum. As the drum is submerged and filtrate fills the ribs, a filtrate air mixture, e.g., foam, occurs in the ribs and can flow into the filtrate chamber 28. The residual air and foam in the ribs should not be drawn into the filtrate chamber, trunnion conduit, elbow conduit and down leg conduit as suction is applied to the ribs. However, when suction is reapplied as the outlet of the ribs rotate past the 5:00 position, the residual air and foam in the ribs flow into the filtrate chamber. This air and foam may be sufficient to reduce the suction created by the drop leg, and create air bubbles in the trunnion.
Air in a washer is detrimental because: (i) when the air is in the filtrate and the cake, it creates resistance to the flow of filtrate through the cake; (ii) air entrained in the filtrate and cake creates foam that is very stable and the foam must typically be eradicated with a costly defoaming agent, and (iii) air in the drop leg results in a lower vacuum created by the drop leg thereby reducing the motive force by which the washer operates.
Prior attempts to vent gases from the filtrate have included adding a gas vent slot in the valve segment that is in fluid communication with the inner conduit formed by the hollow center shaft 38. See e.g., U.S. Pat. No. 5,264,138. The slot may be aligned with the 3:00 to 5:00 position on the drum such that as the channels and ribs rotate down into the vat, the filtrate entering the ribs forces air into the slot and out through the center shaft (rather than into the filtrate chamber and trunnion conduit). The center shaft has a gas vent and a filtrate drain that extends externally of the elbow. The center shaft removes gases in the ribs that would have otherwise entered the elbow. The filtrate drain on the center shaft removes liquid filtrate that enters the hollow shaft with the gases. The gas vent removes gases from the filtrate that are directed into the center shaft. A difficulty with this approach to venting gases is that the center shaft is elevated at or above the liquid level of the vat such some of the air and foam remain in the ribs. The vat fills the ribs with filtrate liquid only to a level in the ribs that is no higher than the vat level. The gap in the ribs between the vat liquid level and center shaft 38 remains filled with air. Another difficulty with the slot open to the center shaft is that the slot is relatively narrow, e.g., 16 degrees, and the center shaft is narrow. The narrow slot and center shaft may not be sufficient to allow gas and foam to vent from the ribs, especially if the drum rotates relatively fast, e.g., above 3 RPM. Another approaches to providing a gas vent for a rotary drum filter include the LaVally valve shown in, for example, U.S. Pat. No. 4,683,059, and the air inflow restrictors shown in U.S. Pat. Nos. 5,683,582 and 5,503,737. However, there remains a long felt need for improved devices and methods for venting gases before they enter the elbow and down leg conduits of a rotary drum filter.